Use unified CPU metric description on OpenBSD (#810)

collector/cpu_openbsd.go

@@ -39,11 +39,7 @@ func init() {
 
 func NewCpuCollector() (Collector, error) {
 	return &cpuCollector{
-		cpu: typedDesc{prometheus.NewDesc(
-			prometheus.BuildFQName(namespace, "cpu", "seconds_total"),
-			"Seconds the CPU spent in each mode.",
-			[]string{"cpu", "mode"}, nil,
-		), prometheus.CounterValue},
+		cpu: typedDesc{nodeCpuSecondsDesc, prometheus.CounterValue},
 	}, nil
 }